In the development of modern passenger and freight railcar equipment, it has been found desirable to reduce free slack which develops from wear on the car's coupling system components that are mounted within the car center sill. Reduction of wear and the resulting slack on these components, particularly on the follower block, will either eliminate or greatly reduce costly sill side wall repairs.
Coupler assemblies are well known in the art and are independent units mounted on each adjacent railcar end for interconnection with one another to form a connection. They are located on the car undercarriage and are attached within the car's center sill structure. Besides holding the cars together, the coupler assemblies function to transmit generally longitudinally-directed forces to the car understructure.
In any car using a center sill, a cast yoke member is retained within the center sill cavity and it plays an important part in transferring the forces from the coupler head into the car understructure. Yoke castings typically include an upper and lower yoke strap member connected to a front and rear yoke wall, in order to form a longitudinal pocket. Yoke castings are well known in the industry and have been standardized dimensionally by the American Association of Railroads (AAR). Also standardized by the AAR are the couplers which are partially mounted within the yoke casting pocket. Typically, the coupler portion of a coupler system is a cast member consisting of an elongate shank portion. A head portion is formed at one end of the shank and a butt end portion is formed at the other end. Typically, the coupler portion can either be a fixed or rotary system, with the butt end portions of each system varying slightly in shape and method of attachment to the yoke casting. In a fixed system, the butt end is typically pinned to the yoke, while in the rotary system, the butt end is typically held inside the yoke by specially shaped retention keys.
It is also another standard of the AAR that a follower block be retained within the yoke casting pocket between the top and bottom yoke straps, internally of the coupler rear butt end portion. The follower block serves as a large bearing surface for communicating the large forces experienced by the coupler head into other coupling system members for final communication into the car understructure. The follower block is generally a solid member having a rectangular shape and the front face of the follower is held against the butt end of the coupler by a draft gear arrangement biasing the rear side of the follower block. The draft gear is typically a frictional energy absorbing and dissipating member which protects the car understructure from the impact forces imparted longitudinally on the coupler. The follower block and draft gear assembly are designed to coaxially and longitudinally move in unison with each other within the yoke casting pocket each time the coupler experiences an external force. In this manner, in response to draft (pulling), and buff (pushing) forces, particularly buff forces, the forces exerted against the coupler are transmitted through the butt end portion directly into the follower block where they are then transmitted into the draft gear and thereby cushioned. The forces absorbed by the draft gear are likewise transmitted into a pair of stationary rear center sill stops. Draft forces are transmitted essentially the same way, except that the follower block functions to transfer the forces pulling on the yoke casting into the center sill side walls through contact with a pair of stationary front center sill stops. The draft assembly now functions to keep the follower biased in place against the butt end portion of the coupler.
The lateral forces experienced by the coupler are not as severe as the longitudinal forces and they are transferred into the center sill in a much different fashion. Since lateral forces tend to push the coupler head in a lateral direction with respect to the longitudinal centerline of the car, the butt end of the coupler also moves, but in the opposite, lateral direction relative to the coupler head. This also means that when the coupler head end is laterally moved, the follower block also moves laterally in unison with the butt end, since the follower block is held against the butt end by the biasing means of the draft gear.
One of the problems with known follower blocks is that the AAR approved follower is designed to have large tolerances between the side walls of the block and the center sill side walls. More particularly, the follower is free to laterally travel within the center sill cavity until it makes contact with the center sill side walls. When a coupler system is new and initially installed within the car understructure, the clearances between the aforementioned system components, known in the art as controlled slack, are minimal. This is true even when the forces on the coupler are reversed, such as when the car is accelerated or decelerated. Eventually however, the extreme forces experienced by the car will cause wear to occur in the entire coupling system such that small gaps will begin to appear between each of the coupler system components. These gaps are known in the art as free slack, and the cumulative affect of the free and controlled slack is to magnify all impact forces experienced during the acceleration and deceleration of the car. It has been known that the lack of confinement of the follower block within the center sill cavity has proved inadequate because the slack between the center sill side walls and the follower block will cumulatively accelerate the wear between the center side walls and the follower block. The end result is that the harder, cast steel follower block eventually wears the softer, steel center sill side walls. If this condition is not detected, the follower block can wear the center sill side walls thin enough so that the car center sill structure will be weakened in the immediate contact area such that the follower block can wear completely through the center sill side walls. If this happens, the coupling system will no longer function as designed. Ultimately, the railcar will have to be removed from service in order to patch and weld the center sill. This method of repair is both time consuming and very expensive because the car must be removed from service and disassembled. Furthermore, the integrity of the railcar center sill is diminished.
A review of certain known patents also shows that little effort has been made to address the center sill wear problems caused by the lateral follower block movement.
U.S. Pat. No. 1,947,936 to Glascodine discloses an outdated coupling system wherein metal liner plates were mounted against the center sill side walls, with a resilient rubber insert being held between the liner and the center sill walls. The rubber insert was to be protected by the liner plates and was intended to absorb the lateral stresses imparted to the side walls by a front and rear follower block. Each follower block side wall peripherally extended into metal-to-metal engagement with the liner plates such that abrasive metal-to-metal contact was not eliminated, leaving the system vulnerable for destructive metal wearing.
U.S. Pat. No. 5,176,268 to Manley is the only known art to address the lateral movement problem as presented in more modern and AAR-approved coupling assemblies recommended today. In that disclosure, Manley addresses the above-mentioned problems by providing an improved follower block which confines the position of the yoke casting within the center sill. The purpose of this follower is to prevent movement of the coupling system components so that free slack does not develop and eventually cause destruction of the center sill side walls. The follower disclosed in the Manley patent is constructed such that it is wider than the AAR standardized follower, making it essentially span the width of the center sill cavity with much less initial controlled slack than before. Furthermore, the rectangularly shaped follower is provided with an outwardly extending tab at each corner of the follower block. The tabs extend upwardly and downwardly, respectively, from the top and bottom sides of the follower, and each of the tabs are lined with an elastomeric wear insert. The respective top and bottom yoke strap is tightly confined between the respective top and bottom pairs of tabs so that in spite of the application of stresses which normally cause lateral follower and yoke movement, the follower maintains the confined configuration of the follower-yoke-side wall interface. However, such efforts to confine the yoke are not the most effective because of the extreme forces placed upon the relatively small bearing surface area of the tabs. Since free slack eventually develops in all coupling components, the tab wear inserts experience extreme and concentrated stress loading, thereby being prone to wear in a relatively short time period and generating additional free slack. Once the additional free slack is introduced in this arrangement, the wear inserts deteriorate at a cumulatively faster rate and the center sill side walls will eventually experience substantial metal-to-metal frictional contact with the side walls of the follower block since this design does not eliminate the potential for metal-to-metal contact between the follower block and center sill side walls. Once again, the problem of a metal follower block potentially wearing the metal center sill side walls is present.